Non-emissive displays, particularly liquid crystal displays, include either reflective displays or surface light source displays (i.e., transmissive displays), commonly denoted backlit displays. Illustrated in FIG. 1 is a conventional reflective display 100. The conventional reflective display 100 includes a liquid crystal suspension 110 sandwiched between glass plates 120, which are sandwiched between polarizers 130. The glass plates 120 can include color filters, common electrodes, TFT matrix, or other components. The conventional reflective display 100 further includes a reflective layer 140 positioned at the bottom of the stack to redirect light back through the other display elements. In operation, light 150 from an ambient source (e.g., sunlight, artificial light (office lighting)) or light 150 from a light source 160 attached to the top of the stack enters the reflective display 100, passes through the polarizers 130, the glass plates 120, and the liquid crystal suspension 140, and is redirected from the reflective film 150 back through the same layers to produce an image. This display 100 creates an image with available ambient light is limited by the available light. This display 100 is not very effective in producing high quality graphic images and severely limits the quality of color images in a variety of conditions.
Illustrated in FIG. 2 is a conventional backlit display 200. The conventional backlit display 200 includes a liquid crystal suspension 210 sandwiched between glass plates 220, which are sandwiched between polarizers 230. The glass plates 220 can include color filters, common electrodes, TFT matrix, or other components. The conventional backlit display 200 further includes a backlight 240 positioned at the bottom of the stack to produce light 250 and direct it through the layers in the stack. Since this device 200 produces an image with artificial light, it is somewhat limited by the amount of ambient light and, in displays where a battery is used some or all of the time to generate power, the battery life. When ambient light is present, glare is created by light reflecting off the various layers, as described above, without passing through all the layers in the stack. To overcome this glare and to produce an image that is palatable to a user, the backlight gain should be increased to produce more usable light, i.e. more light passing through the layers in the stack. This increase in artificial light can cause an added drain on the battery and, thus, reduces the usability of the system to which the display is attached. As ambient light increases, glare increases and, thus, at some point, the backlight becomes ineffective in producing a palatable image.
Previous attempts to use simultaneously the ambient light and a backlight have resulted in applications that compromise both the transmissive qualities and the reflective qualities of the display. Hochstrate, in U.S. Pat. No. 4,196,973 discloses the use of a transflector for this purpose. Weber, in U.S. Pat. No. 5,686,979, discloses the limitations of the transflector for this purpose and alternatively proposes a switchable window that at one time is wholly transmissive and at another time is wholly reflective.